On Tue, Mar 4, 2025 at 11:09 PM Joe Damato <jdam...@fastly.com> wrote:
>
> On Mon, Mar 03, 2025 at 04:03:55PM -0800, Jakub Kicinski wrote:
> > On Mon, 3 Mar 2025 13:33:10 -0500 Joe Damato wrote:
> > > > > @@ -2880,6 +2880,13 @@ static void refill_work(struct work_struct
> > > > > *work)
> > > > > bool still_empty;
> > > > > int i;
> > > > >
> > > > > + spin_lock(&vi->refill_lock);
> > > > > + if (!vi->refill_enabled) {
> > > > > + spin_unlock(&vi->refill_lock);
> > > > > + return;
> > > > > + }
> > > > > + spin_unlock(&vi->refill_lock);
> > > > > +
> > > > > for (i = 0; i < vi->curr_queue_pairs; i++) {
> > > > > struct receive_queue *rq = &vi->rq[i];
> > > > >
> > > >
> > > > Err, I suppose this also doesn't work because:
> > > >
> > > > CPU0 CPU1
> > > > rtnl_lock (before CPU0 calls disable_delayed_refill)
> > > > virtnet_close refill_work
> > > > rtnl_lock()
> > > > cancel_sync <= deadlock
> > > >
> > > > Need to give this a bit more thought.
> > >
> > > How about we don't use the API at all from refill_work?
> > >
> > > Patch 4 adds consistent NAPI config state and refill_work isn't a
> > > queue resize maybe we don't need to call the netif_queue_set_napi at
> > > all since the NAPI IDs are persisted in the NAPI config state and
> > > refill_work shouldn't change that?
> > >
> > > In which case, we could go back to what refill_work was doing
> > > before and avoid the problem entirely.
> > >
> > > What do you think ?
> >
> > Should work, I think. Tho, I suspect someone will want to add queue API
> > support to virtio sooner or later, and they will run into the same
> > problem with the netdev instance lock, as all of ndo_close() will then
> > be covered with netdev->lock.
> >
> > More thorough and idiomatic way to solve the problem would be to cancel
> > the work non-sync in ndo_close, add cancel with _sync after netdev is
> > unregistered (in virtnet_remove()) when the lock is no longer held, then
> > wrap the entire work with a relevant lock and check if netif_running()
> > to return early in case of a race.
>
> Thanks for the guidance. I am happy to make an attempt at
> implementing this in a future, separate series that follows this
> one (probably after netdev conf in a few weeks :).
>
> > Middle ground would be to do what you suggested above and just leave
> > a well worded comment somewhere that will show up in diffs adding queue
> > API support?
>
> Jason, Michael, et. al.: what do you think ? I don't want to spin
> up a v6 if you are opposed to proceeding this way. Please let me
> know.
>
Maybe, but need to make sure there's no use-after-free (etc.
virtnet_close() has several callers).
Thanks
